This invention relates to compositions of matter classified in the art of chemistry as bis-, tri- and higher poly-maleimides, as bis-, tri- and higher poly-citraconimides, as silicone elastomers, as p-phenylenediamine based antiozonants and as sulfur containing organic compounds which are accelerators for the sulfur curing (crosslinking) of polymers which are curable/crosslinkable by sulfur and also sulfur compounds which are polysulfide polymers. The invention also relates to compositions containing them, to processes for their use and to the products produced by such processes.
Polymers and copolymers crosslinked with free radical initiators, organic peroxides and/or azo initiators, are known to have superior properties, particularly to polymers crosslinked by sulfur cure. These properties include high heat ageing resistance, low compression set, decreased staining of metal or coated metal sheet and easy production of colored products which have color stability during crosslinking and during long periods of use. These properties make use of peroxide cure of great practical importance particularly because crosslinking is through a carbon carbon bond rather than through a sulfur containing linkage and this bonding difference is responsible for the improved heat aging and compression set. The drawback for cure of polymers with free radicals from organic peroxides and azo initiators has always been that if air is not excluded from the surface of the material during cure, a tacky surface due to cure inhibition by the molecular oxygen in the air results.
In order to avoid tacky surfaces on objects fabricated using such free radical crosslinking by organic peroxides and/or azo initiators, it has been conventional to exclude air from contact with the surface during cure to avoid the cure inhibition caused by the presence of the molecular oxygen found in atmospheric air. Measures to exclude molecular oxygen add to the cost and complexity of the cure step and sometimes it is difficult, as in the cases of cure in steam autoclaves and in the interior of hoses, to assure the complete exhaustion of air and its molecular oxygen component. In some cases the manufacturer would like to switch from sulfur to peroxide cure and use existing hot air oven curing chambers. Curing with conventional peroxide systems under these circumstances would not be viable as a tacky surface would result.
In order to simplify and reduce the cost and complexity of the cure step, various methods have been suggested for preventing surface cure inhibition by molecular oxygen during free radical crosslinking. These methods have, for various reasons, met with little or no success in actual practice. In particular, none have provided a tack free surface while providing the most desirable physical property of peroxide (azo) cure; superior compression set at 150xc2x0 C. for 70 hours, compared to about 100xc2x0 C., i.e. lower temperature performance for the prior art.
U.S. Pat. No. 4,983,685 discloses the use of compounds selected from the following classes: (a) imidazole compounds, (b) thiourea compounds, (c) thiazole compounds, (d) thiuram compounds, (e) dithiocarbamate compounds, (f) phenol compounds, (g) triazole compounds and (h) amine compounds which are accelerators for sulfur vulcanization in the optional presence of antioxidants, anti-ageing compounds and the like for elastomers for reducing surface tack in the peroxide cure of elastomers in the presence of molecular oxygen. Among optional ingredients which are suggested as possible ingredients for inclusion in the formulations, in this case for increased crosslinking, is N,Nxe2x80x2-m-phenylene bismaleimide.
This is not the preferred optional coagent as a dimethacrylate compound is actually used in the examples. There is no recognition that this latter bismaleimide compound might provide an enhanced effect on the ability of certain compounds of (a) through (h) to reduce surface tack during free radical cure in the presence of molecular oxygen. The use of the various sulfur is accelerators, in particular with peroxide, does provide tack free or reduced tack surfaces for the cured polymers in this reference but the important physical properties expected from a peroxide cure are also reduced. There is no recognition in U.S. Pat. No. 4,983,685 that if the silicone elastomers, bismaleimides and biscitraconimides of the present invention are used in combination with p-phenylene diamine based antiozonants, sulfur containing sulfur vulcanization accelerators and antioxidants and/or polysulfide polymers in free radical cures of polymers, that tack free surfaces and improved physical properties will result and that of all the crosslinking aids mentioned, only these particular classes of compounds have that effect.
Japanese Published Patent Application No. Hei 9[1997]-169873 discloses that antioxidants of the benzimidazole type and of the polymeric 2,2,4-trimethyl-1,2-dihydroquinoline type used in combination with standard crosslinking aids such as methacrylate esters, triallylcyanurates and maleimides, such as this present invention""s preferred component N,Nxe2x80x2-m-phenylene bismaleimide, and standard crosslinking peroxides will result in cured peroxide crosslinkable elastomers with a tack free surface in the presence of air. The inclusion of p-phenylene diamine based antiozonants, silicone elastomers, sulfur accelerators of any type and/or polysulfide polymers is not suggested.
U.S. Pat. No. 4,334,043 teaches the use of surface treatment of curable polymer compositions with organo-metallic compounds, inorganic metallic salts, or lanthanides prior to crosslinking with organic peroxide crosslinking initiators in air to prevent surface tack after crosslinking. Other means of controlling surface tack are not mentioned except for the previously known techniques for surface tack free curing by simply excluding air contact with the rubber surface.
U.S. Pat. Nos. 4,814,384 and 4,973,627 disclose cures of rubber blends for tire treads and sidewalls using a combination of sulfur and peroxide cures. Sulfur accelerators are also employed. Coagents of any type are not mentioned, nor is cure in the presence of air discussed. The use of elemental sulfur is required in the practice of these inventions. We have found, however, that the use of elemental sulfur adversely affects the final physical properties of the cured elastomer to the point where they are more typical of sulfur cure than peroxide cure.
U.S. Pat. No. 4,743,656 also discloses a mixed sulfur/peroxide cure agent for elastomers, which cure agent also includes sulfur accelerators as well as elemental sulfur and the peroxide. Coagents are not mentioned, nor are crosslinking in air and surface tackiness discussed.
U.S. Pat. No. 4,575,552 claims the use of specific combinations of hindered phenol antioxidants, metal salts of dithiocarbamates and m-phenylene-dimaleimide to provide a peroxide crosslinked polymer with superior hydrolytic and thermal stability for geothermal applications. There is no mention of crosslinking in the presence of air, air-inhibition or surface tackiness as a result of air-inhibition.
U.S. Pat. No. 5,849,214 discloses the use of sulfur compounds, sulfur accelerators and hydroquinones with the optional presence of crosslinking aids (coagents) in the retardation of scorch during compounding of free radical crosslinkable polymers in the presence of free radical initiators. Bismaleimides, and biscitraconimides are not specifically discussed nor is there any mention of the possible effect on surface tackiness during cure in the presence of molecular oxygen (air) for any of the compositions disclosed.
In addition there are patents which use various compounds to physically coat the surface of crosslinkable elastomers to exclude air (oxygen) e.g., U.S. Pat. No. 4,439,388 which teaches use of boric acid, boric acid anhydride as a surface treatment prior hot air cure. This surface coating technique is labor intensive as it must be removed and disposed of after the crosslinking reaction step is completed.
None of the above references, taken singly or in combination, suggests applicants"" solution described and claimed herein for elimination of surface tack due to air-inhibition during free radical cure of polymers by free radical curing agents, such as organic peroxides and azo initiators while providing the desirable physical properties of a peroxide (azo) cure. For example, the compression set values expected from a standard peroxide cure and wherein the compression set is measured at temperatures in the region about 150xc2x0 C. for 70 hours.
The invention provides in its first composition aspect, a composition comprising:
a) At least one compound (A) selected from the group consisting of silicone elastomers and a compound having the formula (I): 
xe2x80x83wherein n is 1, or 2 and R is divalent, or trivalent and is selected from the group consisting of acyclic aliphatic groups having from about 2 to 16 carbon atoms, cyclic aliphatic groups having from about 5 to 20 carbon atoms, aromatic groups having from about 6 to 18 carbon atoms and alkyl aromatic groups having from about 7 to 24 carbon atoms, and wherein those divalent, or trivalent groups may contain one or more heteroatoms selected from O, N and S, replacing a carbon atom, or atoms, and each R1 is identical and is hydrogen or an alkyl group of 1 to 18 carbon atoms; and
(b) at least one compound (B) selected from the group consisting of p-phenylenediamine based antiozonants and sulfur containing organic compounds selected from the group consisting of sulfur containing organic compounds capable of accelerating sulfur vulcanization of polymers capable of being crosslinked by sulfur (xe2x80x9csulfur acceleratorsxe2x80x9d), polysulfide polymers and mixtures of said sulfur containing compounds.
The tangible embodiments of the first composition aspect of the invention possess the inherent applied use characteristic of being suppressors of surface inhibition of free radical induced cure of polymers in the presence of gaseous molecular oxygen, (i.e., oxygen present in the atmosphere) thereby permitting tack free cures of polymers by free radical curing agents in the presence of air while maintaining the final physical properties associated with a conventional peroxide cure.
The invention provides in a subgeneric aspect of the first composition aspect of the invention, a composition formed by mixing as the essential ingredients thereof at least one member of compound (A) and at least one member of compound (B) of the first composition aspect of the invention.
The invention provides in a second composition aspect, a composition comprising a composition as defined in the first composition aspect and a free radical initiator selected from the group consisting of organic peroxides and azo initiators.
The tangible embodiments of the second composition aspect of the invention possess the inherent applied use characteristic of being curing or crosslinking agents for those polymers capable of being crosslinked by free radical initiators and of being capable of effecting such cures in the presence of air (molecular oxygen) without the polymer being cured experiencing surface inhibition by the presence of air (molecular oxygen), thus, providing a cured or crosslinked polymer having a substantially tack free surface without the necessity for avoiding contact of said surface with air (molecular oxygen) during cure.
The invention provides a subgeneric aspect of the second composition aspect of the invention. Said composition being one prepared by mixing in any order, at least one member of compound (A) and at least one member of compound (B) of the first composition aspect of the invention and a free radical initiator as defined for the second composition aspect of the invention.
The invention provides in a third composition aspect a curable composition comprising a polymer curable by free radical initiators and a composition as defined in the second composition aspect of the invention.
The third composition aspect of the invention possesses the inherent applied use characteristic of being formable into a shaped article and then being crosslinkable while the surface of said shaped article is in contact with air (molecular oxygen) to provide a crosslinked shaped article having a substantially tack free surface.
The invention provides in a subgeneric composition aspect of the third composition aspect of the invention a curable composition prepared by mixing in any order at least one member of compound (A) and at least one member of compound (B) of the first composition aspect of the invention, a free radical initiator as defined for the second composition aspect of the invention and a polymer crosslinkable by a free radical initiator.
The invention provides in a first process aspect a process for the preparation of the second composition aspect of the invention which comprises mixing in any order at least one member of compound (A) and at least one member of compound (B) of the first composition aspect of the invention and a free radical initiator as defined in the second composition aspect of the invention.
The invention provides in a second process aspect of the invention, a process for the preparation of the third composition aspect of the invention which comprises mixing in any order at least one member of compound (A) and at least one member of compound (B) of the first composition aspect of the invention, a free radical initiator as defined in the second composition aspect of the invention and a polymer crosslinkable by a free radical initiator.
Special mention is made of embodiments of the several aspects of the invention wherein compound (A) is selected from bismaleimides and compound (B) is selected from sulfur accelerators, where compound (A) is selected from biscitraconimides and compound (B) is selected from sulfur accelerators, where compound (A) is selected from bismaleimides and compound (B) is selected from polysulfide polymers, where compound (A) is selected from biscitraconimides and compound (B) is selected from polysulfide polymers, where compound (A) is selected from silicone elastomers and compound (B) is selected from polysulfide polymers.
Special mention is also made of embodiments of the several aspects of the invention wherein chlorinated polyethylene and/or chlorosulfonated polyethylene are included as optional supplemental ingredients in addition to compounds (A) and (B).
The best mode contemplated by the inventors for making and using their invention will now be described in detail with reference to a particular embodiment thereof, namely:
A mixture of dipentamethylene thiuram tetra-sulfide (Sulfads), N,Nxe2x80x2-m-phenylene bismaleimide (HVA-2) and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane (LUPEROX(copyright) 231 XL) used to cure ethylene propylene copolymer (VISTALON(copyright) 504) in hot air.
To prepare the mixture of Sulfads, HVA-2 and LUPEROX 231 XL, the ingredients, which are all in dry powder form (the LUPEROX(copyright) 231 XL is in the form of 40% by weight peroxide dispersed on calcium carbonate), may be mixed in any order and then compounded by standard methods (Banbury, two roll mill, extruder and the like) into the VISTALON(copyright) polymer. The Sulfads(copyright), HVA-2 and LUPEROX 231 XL may also be compounded directly into the VISTALON either simultaneously or sequentially in any order. Any two of the Sulfads, HVA-2 and LUPEROX 231 XL ingredients may be mixed and compounded into the VISTALON separately or simultaneously with the third ingredient. This compounding, if done separately, may also be performed in any order of s ingredient addition to the polymer, but it is preferred if the peroxide is added last.
Once compounding with the VISTALON is complete, the compounded mixture may be cured simply by placing it in a hot air oven at a suitable temperature for initiating cure by decomposition of the peroxide, conveniently, in this case, at about 365xc2x0 F. (about 185xc2x0 C.), for a sufficient period of time to permit the desired degree of crosslinking to take place, conveniently, in this case, about minutes, for a thin sample at room temperature at the start.
One of skill in the art will recognize that the other compounds falling within the scope of Formula I of the first composition aspect of this invention are all solid materials, are all trimaleimides, bismaleimides, tricitraconimides, or bis citraconimides and can all be combined with the other starting materials contemplated by the invention by similar conventional methods to those described. The bismaleimides and biscitraconimides contemplated as starting materials are all either commercially available or can be readily synthesized by methods well known in the art. See, for example, U.S. Pat. No. 5,494,948; 5,616,666, 5,292,815 and the references cited therein for more general synthetic methods.
The trimaleimides and tricitraconimides as well as the higher polymaleimides and citraconimides may be prepared by analogous techniques if they are not commercially available. For example, the trimaleimide, N,Nxe2x80x2,Nxe2x80x3-(1,3,5-triazine-2,4,6-triyl)trimaleimide has CAS number CAS(67460-81-5).
Some primary amines suitable for synthesis of the di, tri- and higher polymaleimides and analogous citraconimides are polyfunctional primary amines such as melamine and the various polyoxypropylene amines such as the polyoxypropylene diamines and the polyoxypropylene triamines sold under the JEFFAMINE tradename by Huntsman Corporation.
In addition to the N,Nxe2x80x2-m-phenylene-bismaleimide specifically referenced above, other bismaleimides, in addition to those disclosed in the above referenced patents, suitable for use in the invention, without limiting the generality of the above general formula (I), are:
N,Nxe2x80x2-ethylenebismaleimide, N,Nxe2x80x2-hexamethylenebismaleimide, N,Nxe2x80x2-dodecamethylene-bismaleimide, N,Nxe2x80x2-(2,2,4-trimethylhexamethylene)bismaleimide, N,Nxe2x80x2-(oxy-dipropylene)bismaleimide, N,Nxe2x80x2-(aminodipropylene)bismaleimide, N,Nxe2x80x2-(ethylenedioxy-dipropylene)bismaleimide, N,Nxe2x80x2(1,4-cyclohexylene)bismaleimide, N,Nxe2x80x2-(1,3-cyclohexylene)bismaleimide, N,Nxe2x80x2-(methylene 1,4-dicyclohexylene)bismaleimide, N,Nxe2x80x2-(isopropylidene-1,4-dicyclohexylene)bismaleimide, N,Nxe2x80x2-(oxy-1,4-dicyclohexylene)bismaleimide, N,Nxe2x80x2-p-(phenylene)bismaleimide, N,Nxe2x80x2-(o-phenylene)bismaleimide, N,Nxe2x80x2-(1,3-naphthylene)bismaleimide, N,Nxe2x80x2-(1,4-naphthylene)bismaleimide, N,Nxe2x80x2(1,5-naphthylene)bismaleimide, N,N-(3,3xe2x80x2-dimethyl-4,4xe2x80x2-diphenylene)bismaleimide, N,Nxe2x80x2-(3,3-dichloro-4,4xe2x80x2-biphenylene)bismaleimide, N,Nxe2x80x2-(2,4-pyridyl)bismaleimide, N,Nxe2x80x2-2,6-pyridyl)bismaleimide, N,Nxe2x80x2-(1,4-anthraquinonediyl)bismaleimide, N,Nxe2x80x2-(m-tolylene)bismaleimide, N,Nxe2x80x2-(p-tolylene)bismaleimide, N,Nxe2x80x2-(4,6-dimethyl-1,3-phenylene)bismaleimide, N,Nxe2x80x2-(2,3-dimethyl-1,4-phenylene)bismaleimide, N,Nxe2x80x2-(4,6-dichloro-1,3-phenylene)bismaleimide, N,Nxe2x80x2-(5-chloro-1,3-phenylene)bismaleimide, N,Nxe2x80x2-(5-hydroxy-1,3-phenylene)bismaleimide, N,Nxe2x80x2-(5-methoxy-1,3-phenylene)bismaleimide, N,Nxe2x80x2-(m-xylylene)bismaleimide, N,Nxe2x80x2-(p-xylylene)bismaleimide, N,Nxe2x80x2-(methylenedi-p-phenylene)bismaleimide, N,Nxe2x80x2-(isopropylidenedi-p-phenylene)bismaleimide, N,Nxe2x80x2-(oxydi-p-phenylene)bismaleimide, N,Nxe2x80x2-(thiodi-p-phenylene)bismaleimide, N,Nxe2x80x2-(dithiodi-p-phenylene)bismaleimide, N,Nxe2x80x2-(sulfodi-p-phenylene)bismaleimide, N,Nxe2x80x2-(carbonyldi-p-phenylene)bismaleimide, xcex1,xcex1-bis-(4-maleimodophenyl)-meta-diisopropylbenzene, xcex1,xcex1-bis-(4-p-phenylene)bismaleimide and xcex1,xcex1-bis-(4-maleimidophenyl)para-diisopropylbenzene.
Combination of two or more bismaleimides, or bismaleimides with the trimaleimides, and with the higher polymaleimides in the compositions and processes of the invention are also contemplated as equivalents and one of skill in the art would understand that such tri and higher polymaleimides and their substitution for the compounds and processes specifically illustrated herein for the practice of the invention to be such equivalents and to be well within the scope contemplated by the invention.
Biscitraconimides, which may be substituted in whole or in part for the N,Nxe2x80x2-m-phenylenebismaleimide referenced above include as representative examples:
1,2-N,Nxe2x80x2-dimethylene biscitraconimide;
1,2-N,Nxe2x80x2-trimethylene biscitraconimide;
1,5-N,Nxe2x80x2-2-methyl-pentamethylene)-biscitraconimide; and
N,Nxe2x80x2-methylphenylene biscitraconimide.
Mixtures of biscitraconimides and mixtures of bismaleimides and biscitraconimides as well as those including the trimaleimides are also contemplated as equivalents by the invention.
The biscitraconimides contemplated by the invention are all well known compounds and where not commercially available, they may be readily synthesized by methods detailed in the art. U.S. Pat. No. 5,292,815 in column 4, provides a detailed list of such methods. As stated above, the tri- and higher polycitraconimides may be prepared by analogous methods and substituted in whole or in part in the compositions of the invention and such compounds and substitutions will be understood by one of skill in the art as being a full equivalent to those specifically illustrated herein and well within the scope contemplated as equivalent by the invention.
The silicone elastomers contemplated as useful in the aspects of the invention are the peroxide crosslinkable dimethyl vinyl substituted silicone derivative elastomers which are well known in the art. See, for example, xe2x80x9cKirk Othmer Encyclopedia of Chemical Technologyxe2x80x9d, Vol. 20, pp. 943 et seq., John Wiley and Sons, (copyright) 1982.
Sulfur containing organic compounds capable of accelerating sulfur vulcanization of polymers, which are capable of being crosslinked by sulfur contemplated for use in the invention are well known in the art. Many different classes of these compounds are known and all are contemplated as equivalent.
The Vanderbilt Rubber Handbook, thirteenth edition, 1990, R.T. Vanderbilt Company, Inc., publisher lists many types. Illustrative of these are derivatives of benzothiazoles, thiadiazoles, sulfenamides, sulfenimides, dithiocarbamates, thiurams, imidazoles, xanthates, and thioureas. Also included in this general class of sulfur compound sulfur accelerators are sulfides, disulfides (e.g., diallyldisulfide) polysulfides and arylpolysulfide compounds such as the amylphenol polysulfides e.g. VULTAC(copyright) products from ATOFINA Chemicals, Inc. and other sulfides such as disulfide and/or other known sulfur accelerating polysulfide phosphate, dithiophosphates and/or phosphorous and sulfur containing compounds. Other sulfur containing organic compounds capable of sulfur donation at vulcanization temperatures which are known but are not presently used for such reactions because of cost concerns are also contemplated as equivalents. Illustrative of these is the compound 2-(2,4-cyclopentadiene-1-ylidene)-1,3-dithiolane.
More particularly, one sulfur accelerator class suitable for use in the practice of the invention are salts of disubstituted dithiocarbamic acid.
These salts have the general structure: 
Wherein X is an ion derived from a metal selected from the group consisting of nickel, cobalt, iron, chromium, tin, zinc, copper, lead, bismuth, cadmium, selenium and tellurium, or X is a quaternary ammonium ion, n may vary from 1 to 6 and is equal to the number of formal positive charges on the X ion, and R1 and R2 are independently alkyl of 1 to 7 carbon atoms.
Examples of the salts of disubstituted dithiocarbamic acid are:
bismuth dimethyldithiocarbamate;
cadmium diethyldithiocarbamate;
cadmium diamyldithiocarbamate;
copper dimethyldithiocarbamate;
lead diamyldithiocarbamate;
lead dimethyldithiocarbamate;
selenium diethyldithiocarbamate;
selenium dimethyldithiocarbamate;
tellurium diethyldithiocarbamate;
piperidinium pentamethylene dithiocarbamate;
zinc diamyldithiocarbamate;
zinc diisobutyldithiocarbamate
zinc diethyldithiocarbamate;
zinc dimethyldithiocarbamate;
copper dibutyldithiocarbamate;
sodium dimethyldithiocarbamate;
sodium diethyldithiocarbamate;
sodium dibutyldithiocarbamate;
zinc di-n-butyldithiocarbamate;
zinc dibenzyldithiocarbamate.
A second sulfur accelerator class suitable for use in the invention comprises the thiurams. These are prepared from secondary amines and carbon disulfide and possess the general structure: 
Wherein R3 is an alkyl group of from 1 to about 7 carbon atoms or the R3 groups on each particular nitrogen atom may be concatenated to form, together with the nitrogen atom on which they are attached, a five, six or seven membered heterocyclic ring containing 4, 5 or 6 carbon atoms respectively and n may have a positive value from greater than zero up to 6.
Typical examples of thiuram sulfur accelerators are:
dipentamethylenethiuram tetrasulfide and hexasulfide;
tetrabutylthiuram disulfide;
tetramethylthiuram disulfide;
tetraethylthiuram disulfide;
tetramethylthiuram monosulfide;
isobutylthiuram disulfide;
dibenzylthiuram disulfide;
tetrabenzylthiuram disulfide;
tetraisobutylthiuram disulfide;
isobutylthiuram monosulfide;
dibenzylthiuram monosulfide;
tetrabenzylthiuram monosulfide;
tetraisobutylthiuram monosulfide.
The higher multisulfides of the various thiurams are also sulfur donors.
Derivatives of thiadiazoles are, but not limited to, monobenzoyl derivatives of dimercaptothiadiazole (2,5-dimethyl-1,3,4-thiadiazole); the proprietary thiadiazole of the Vanderbilt Rubber Company identified as VANAX(copyright) 189; 1,2,4-thiadiazole, 5-ethoxy-3-(trichloromethyl)thiadiazole; and alkyl mercaptothiadiazoles, e.g. methyl mercapto thiadiazole.
Derivatives of benzothiazoles have the general structure: 
Wherein M is a direct bond between two sulfur atoms, H, or an ion derived from a metal selected from the group consisting of nickel, cobalt, iron, chromium, tin, zinc, copper, lead, bismuth, cadmium, selenium and tellurium; and when M is H, x is 1; when M is a direct bond between two sulfur atoms, x is 1 or 2; and when M is an ion derived from a metal, x is equal to the formal valence of the metal ion; and if M is a direct bond between two sulfur atoms and x is 1, then the second sulfur atom to which the M bond is attached is also bonded to a 4-morpholinyl radical.
Illustrative compounds are: 2-(4-morpholinodithio) benzothiazole; benzothiazyl disulfide; 2-mercapto-benzothiazole; 2-mercaptobenzothiazole disulfide; sodium-2-mercaptobenzothiazolate; zinc-2-mercapto-benzothiazole; copper-2-mercaptobenzothiazolate; 2-N-cyclohexylaminobenzothiazole; N-cyclohexylamino-2-benzothiazole polysulfide; 2-bisbenzothiazole-2,2-polysulfide and 2-bisbenzothiazole-2,2-disulfide; bis(2,2xe2x80x2-benzothiazyldisulfide).
The sulfenamide accelerators are also well known. Illustrative examples are: N-oxydiethylene-2-benzothiazole sulfenamide; N-oxydiethylene thiocarbamyl-N-oxydiethylene sulfenamide; N-cyclohexyl-2-benzothiazole sulfenamide; N-t-butyl-2-benzothiazole sulfenamide; N-cyclohexyl-2-benzothiazylsulfeneamide; N,N-dicyclohexyl benzthiazyl sulfenamide; N-t-butyl-2-benzothiazole sulfenamide. There are also sulfenimide compounds, e.g., N-t-butyl-benzothiazole-2-sulfenimide.
Typical imidazoles are: 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole; and the zinc salt of 2-mercaptobenzimidazole.
Zinc isopropyl xanthate is a typical xanthate sulfur accelerator.
Typical thioureas are: trimethylthiourea; 1,3-diethylthiourea and 1,3-dibutylthiourea; ethylene thiourea; blend of dialkyl thioureas; diphenyl thiourea; diorthotolyl thiourea; dimethyl thiourea; diethyl thiourea; dibutyl thiourea.
Alkylphenoldisulfide types of sulfur accelerators are illustrated by the compounds available from ATOFINA Chemicals, Inc., under the designation VULTAC(copyright) 2, VULTAC 3 and VULTAC 5.
Thiophosphate sulfur accelerators are illustrated by such compounds as copper dialkyldithiophosphate; zinc dialkyldithiophosphate; zinc amine dithiophosphate; zinc dibutyldithophosphate; copper O,O-diisopropyl-phosphorodithiolate; zinc O,O-diisopropylphosphorodithiolate.
Other miscellaneous sulfur accelerators include 4,4-dithiodimorpholine; N,Nxe2x80x2-caprolactam disulfide; dibutylxanthogen disulfide.
The polymers which can be cured (crosslinked) in the presence of molecular oxygen include all those natural and synthetic polymers capable of being crosslinked either by abstraction of hydrogen (or other extractable atoms, such as with iodo and bromo substituted fluoroelastomers) or by polymerization through double bonds.
Polymers which are currently understood to not be crosslinkable by these mechanisms and which undergo degradation in the presence of free radicals generated from organic peroxides and the azo initiators defined herein below and whose presence should be substantially avoided in the curable compositions of this invention include: poly(vinyl chloride), poly-(propylene), butyl rubber, epichlorohydrin polymers and epichlorohydrin ethylene oxide polymers. When used herein and in the appended claims when referring to polymers suitable for use in connection with the invention any reference to a group of polymers using the terms xe2x80x9ccomprising,xe2x80x9d xe2x80x9cconsisting essentiallyxe2x80x9d and xe2x80x9cconsisting ofxe2x80x9d expressly excludes more than minor insignificant amounts (1% by weight or less) of the non free radical crosslinkable polymers in the absence of an express statement to the contrary.
Polymers crosslinkable by free radicals from organic peroxides and azo initiators as defined herein below include ethylene-propylene terpolymer (EPDM), ethylene-propylene copolymer (EPM) natural polyisoprene rubber (NR), styrene butadiene rubber (SBR), polybutadiene rubber (BR), synthetic polyisoprene rubber (IR), poly(ethylene) (PE), ethylene-vinyl acetate (EVA), acrylonitrile-butadiene-styrene (ABS), unsaturated polyester, styrene-butadiene-styrene block copolymers (SBS), styrene-isoprenestyrene block copolymers (SIS), neoprene rubber (CR), nitrite rubber (NBR), polysulfide rubber (T) chlorinated polyethylene) (CM), polyurethane (AU, EU), vinylidene fluoride copolymers (CFM), silicone rubber (PMQ), vinyl silicone rubber (VMQ, PVMQ), polyacrylate (ACM), chlorosulfonated poly(ethylene) (CSM) and fluorosilicone rubber (FVMQ).
The free radical initiators (organic peroxides and azo initiators) suitable for use in the invention include all those classes of organic peroxides and azo initiators suitable for curing (crosslinking) polymers, both thermoplastics and elastomers.
The azo initiators are those known in the art, such as 2,2xe2x80x2-azobis-(2-acetoxypropane), to generate free radicals on heat decomposition capable of inducing the desired curing (crosslinking) reaction. The azo initiators of U.S. Pat. Nos. 3,862,107 and 4,129,531, the disclosures of which are incorporated herein by reference, are also suitable.
With the exception of hydroperoxides and liquid peroxydicarbonates, all those organic peroxides known to undergo decomposition by heat to generate radicals capable of initiating the desired curing (crosslinking) reactions are contemplated as suitable for use in the invention. Dialkyl peroxides, diperoxyketals, mono-peroxy carbonates, cyclic ketone peroxides, diacyl peroxides, organosulfonyl peroxides, peroxyesters and solid, room temperature stable peroxydicarbonates are the preferred initiators. The most preferred initiators are dialkyl peroxides, peroxyketals, cyclic ketone peroxides and diacyl peroxides.
A good reference which provides important peroxide names and physical properties for all these classes of organic peroxides can be found in xe2x80x9cOrganic Peroxidesxe2x80x9d by Jose Sanchez and Terry N. Myers; Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Ed., Volume 18, (1996).
Illustrative dialkyl peroxide initiators are:
di-t-butyl peroxide;
t-butyl cumyl peroxide;
2,5-di(cumylperoxy)-2,5-dimethyl hexane;
2,5-di(cumylperoxy)-2,5-dimethyl hexyne-3;
4-methyl-4-(t-butylperoxy)-2-pentanol;
4-methyl-4-(t-amylperoxy)-2-pentanol;
4-methyl-4-(cumylperoxy)-2-pentanol;
4-methyl-4-(t-butylperoxy)-2-pentanone;
4-methyl-4-(t-amylperoxy)-2-pentanone;
4-methyl-4-(cumylperoxy)-2-pentanone;
2,5-dimethyl-2,5-di(t-butylperoxy)hexane;
2,5-dimethyl-2,5-di(t-amylperoxy)hexane;
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3;
2,5-dimethyl-2,5-di(t-amylperoxy)hexyne-3;
2,5-dimethyl-2-t-butylperoxy-5-hydroperoxyhexane;
2,5-dimethyl-2-cumylperoxy-5-hydroperoxy hexane;
2,5-dimethyl-2-t-amylperoxy-5-hydroperoxyhexane;
m/p-alpha, alpha-di[(t-butylperoxy)isopropyl]benzene;
1,3,5-tris(t-butylperoxyisopropyl)benzene;
1,3,5-tris(t-amylperoxyisopropyl)benzene;
1,3,5-tris(cumylperoxyisopropyl)benzene;
di[1,3-dimethyl-3-(t-butylperoxy)butyl]carbonate;
di[1,3-dimethyl-3-(t-amylperoxy)butyl]carbonate;
di[1,3-dimethyl-3-(cumylperoxy)butyl]carbonate;
di-t-amyl peroxide;
t-amyl cumyl peroxide;
2,4,6-tri(butylperoxy)-s-triazine;
1,3,5-tri[1-(t-butylperoxy)-1-methylethyl]benzene
1,3,5-tri-[(t-butylperoxy)-isopropyl]benzene;
1,3-dimethyl-3-(t-butylperoxy)butanol;
1,3-dimethyl-3-(t-amylperoxy)butanol; and mixtures thereof.
Illustrative solid, room temperature stable peroxydicarbonates are, but not limited to:
di(2-phenoxyethyl)peroxydicarbonate; di(4-t-butyl-cyclohexyl)peroxydicarbonate; dimyristyl peroxydicarbonate; dibenzyl peroxydicarbonate; di(isobornyl)peroxydicarbonate.
Another preferred class of dialkylperoxides which may be used singly or in combination with the other free radical initiators contemplated by the invention are those selected from the group represented by the formula: 
Wherein R4 and R5 may independently be in the meta or para positions and are the same or different and are selected from the group hydrogen or straight or branched chain alkyl of from 1 to 6 carbon atoms. Dicumyl peroxide and isopropylcumyl cumyl peroxide are illustrative.
Other dialkyl peroxides are:
3-cumylperoxy-1,3-dimethylbutyl methacrylate;
3-t-butylperoxy-1,3-dimethylbutyl methacrylate;
3-t-amylperoxy-1,3-dimethylbutyl methacrylate;
tri(1,3-dimethyl-3-t-butylperoxy butyloxy)vinyl silane;
1,3-dimethyl-3-(t-butylperoxy)butyl N-[1-{3-(1-methylethenyl)-phenyl}1-methylethyl]carbamate;
1,3-dimethyl-3-(t-amylperoxy)butyl N-[1-{3(1-methylethenyl)-phenyl}-1-methylethyl]carbamate;
1,3-dimethyl-3-(cumylperoxy))butyl N-[1-{3-(1-methylethenyl)-phenyl}-1-methylethyl]carbamate.
In the group of diperoxyketal initiators, the preferred initiators are:
1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane;
1,1-di(t-butylperoxy)cyclohexane;
n-butyl 4,4-di(t-amylperoxy)valerate;
ethyl 3,3-di(t-butylperoxy)butyrate;
2,2-di(t-amylperoxy)propane;
3,6,6,9,9-pentamethyl-3-ethoxycabonylmethyl-1,2,4,5-tetraoxacyclononane;
n-butyl-4,4-bis(t-butylperoxy)valerate;
ethyl-3,3-di(t-amylperoxy)butyrate; and mixtures thereof.
Other peroxides falling within the general class defined as useful in the invention include benzoyl peroxide, 00-t-butyl-0-hydrogen-monoperoxy-succinate and 00-t-amyl-0-hydrogen-monoperoxy-succinate.
Illustrative cyclic ketone peroxides are compounds having the general formulae (II), (III) and/or (IV). 
Wherein R1 to R10 are independently selected from the group consisting of hydrogen, C1 to C20 alkyl, C3 to C20 cycloalkyl, C6 to C20 aryl, C7 to C20 aralkyl and C7 to C20 alkaryl, which groups may include linear or branched alkyl properties and each of R1 to R10 may be substituted with one or more groups selected from hydroxy, C1 to C20 alkoxy, linear or branched C1 to C20 alkyl, C6 to C20 aryloxy, halogen, ester, carboxy, nitride and amido, preferably, at least 20% of the total active oxygen content of the peroxide mixture used for a crosslinking reaction will be from compounds having formulas (II), (III) and/or (IV).
Some examples of suitable cyclic ketone peroxides are:
3,6,9, triethyl-3,6,9-trimethyl-1,4,7-triperoxynonane (or methyl ethyl ketone peroxide cyclic trimer) and methyl ethyl ketone peroxide cyclic dimer,
3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxacyclononane;
Illustrative suitable peroxy esters are:
2,5-dimethyl-2,5-di(benzoylperoxy)hexane;
t-butylperbenzoate;
t-butylperoxy acetate;
t-butylperoxy-2-ethyl hexanoate;
t-amyl perbenzoate;
t-amyl peroxy acetate;
t-butyl peroxy isobutyrate;
3-hydroxy-1,1-dimethyl t-butyl peroxy-2-ethyl hexanoate;
00-t-amyl-0-hydrogen-monoperoxy succinate;
00-t-butyl-0-hydrogenmonoperoxy succinate;
di-t-butyl diperoxyphthalate;
t-butylperoxy (3,3,5-trimethylhexanoate);
1,4-bis(t-butylperoxycarbo)cyclohexane;
t-butylperoxy-3,5,5-trimethylhexanoate;
t-butyl-peroxy-(cis-3-carboxy)propionate;
allyl 3-methyl-3-t-butylperoxy butyrate.
Illustrative monoperoxy carbonates are:
00-t-butyl-0-isopropylmonoperoxy carbonate;
00-t-butyl-0-(2-ethyl hexyl)monoperoxy carbonate;
1,1,1-tris[2-(t-butylperoxy-carbonyloxy)ethoxymethyl]propane;
1,1,1-tris[2-(t-amylperoxy-carbonyloxy)ethoxymethyl]propane;
1,1,1-tris[2-(cumylperoxy-cabonyloxy)ethoxymethyl]propane;
OO-t-amyl-O-isopropylmonoperoxy carbonate.
Illustrative diacyl peroxides are:
di(4-methylbenzoyl)peroxide;
di(3-methylbenzoyl)peroxide;
di(2-methylbenzoyl)peroxide;
didecanoyl peroxide; dilauroyl peroxide;
2,4-dibromo-benzoyl peroxide;
succinic acid peroxide.
dibenzoyl peroxide;
di(2,4-dichloro-benzoyl)peroxide.
Imido peroxides of the type described in PCT Application publication WO9703961 A1 6 Feb. 1997 are also contemplated as suitable for use by the invention.
One of skill in the art will readily be able to select suitable quantities of the various ingredients for use in the invention and will quickly and easily be able to optimize the concentrations through a series of bench scale trials employing increasing amounts of the ingredients in samples of the polymer to be cured (crosslinked). The optimum processing (compounding) time and temperatures and the like may also be determined from the same trials as will the optimum cure time and temperature.
Typically, one will employ the compounds of formula (a) (the bismaleimide and biscitraconimides) in the composition of the invention in quantities which will provide from about 0.2 parts by weight per part of polymer by weight (phr) to about 10.0 phr, preferably from about 1.0 phr to about 5.0 phr, most preferably from about 1.5 phr to about 3.0 phr.
Typically, one will employ the sulfur containing organic compound capable of accelerating sulfur vulcanization in polymers capable of being crosslinked by sulfur in compositions of the invention in quantities which will provide from about 0.01 phr to about 20 phr, preferably from about 0.1 phr to about 1.0 phr, most preferably from about 0.1 phr to about 0.5 phr. It is understood by those of skill in the art that these compounds are of two types. Those that donate sulfur to the vulcanization and those which simply accelerate sulfur vulcanization. Either class of compound or mixtures thereof are contemplated as equivalents by the invention.
Alkyl phenol disulfide polymers of the VULTAC(copyright) type are preferably used at from about 0.5 phr to 20 phr when used alone or at from about 0.1 phr to about 10 phr when in combination with other sulfur accelerators.
Typically, one will employ the free radical initiator (organic peroxide and/or azo initiator) in quantities of from about 0.04 to about 10 phr preferably from about 1 to about 4 phr.
The time-temperature conditions necessary for curing largely depend on the structure of the free radical curing agent. For the azo initiators, suitable conditions are detailed in U.S. Pat. No. 3,632,107 and 4,129,531.
For the compositions of the invention, appropriate time and temperature conditions may be determined for crosslinking a particular polymer composition by running a small number of well controlled rheometer studies and selecting values from the results of those studies where the time/temperature relationship is from five to fifteen times the half life value for the free radical initiator in the system.
The invention contemplates that other conventional additives such as anti-oxidants (hindered phenols and polymeric quinoline derivatives are preferred), aliphatic process oils, and other process aids, pigments, dyes, tackifiers, waxes, reinforcing aids, UV stabilization agents, blowing agents and activators and antiozonants may also be present in the compositions before, after and during the curing step.
The polysulfide polymers contemplated by the invention are those known polysulfide polymers which are prepared by the reaction of an xcex1,xcfx89-dihaloalkyl (or dihaloheteroalkyl) compound with a metallic, preferably an alkali metal, polysulfide. The common commercially available polysulfide polymers are either liquids or solids, are either thiol or hydroxy terminated and are derived from materials produce by the reaction of 1,2-dichloroethane, 2,2xe2x80x2dichloro diethyl ether or bis(2-chloroethyl)formal with an alkali metal polysulfide (MSxx) wherein M is an alkali metal ion, preferably derived from sodium and x is a number greater than 1 up to about six.
The invention contemplates that polysulfide polymers may be used in place of or in admixture with the other compounds (B) in equal quantities to those previously specified for those compounds. Since an excess of polysulfide polymer is not contemplated as detrimental to the practice of the invention, it is also contemplated that they may be preblended with the compound(s) (A) and optionally with the free radical initiator(s) to form masterbatches, either solid or liquid. The polysulfide polymers may also be preblended into the polymer to be cured and and the compound(s) (A) and also the free radical initiator(s) blended in simultaneously or subsequently at the option of the operator. Use of the polysulfide polymers in combination with the other sulfur accelerators contemplated by the invention permits reduction of the amount of sulfur accelerator required by the invention.
Similarly it will be obvious to one of skill in the art that polysulfide polymers themselves may be cured to tack free surfaces with free radical initiators in the presence of molecular oxygen if a compound (A) is present in the curable composition even if another compound (B) is not present. Thus, the invention contemplates such a curable composition as an equivalent to the second composition aspect of the invention as defined above.
Certain crosslinkable elastomer compositions which are highly filled with oil and/or carbon black (commonly referred to as highly extended elastomer formulations) are normally cured using sulfur vulcanization rather than free radical initiators. Free radical cure is more difficult because the radicals generated lack specificity and react with the filler and oil as well as the elastomer. This reduces efficiency of the free radical initiator.
It has been found that the use of chlorinated polyethylene and/or chlorosulfonated polyethylene as supplemental ingredients to all the compositions of the various composition aspects of the invention surprisingly increases free radical cure efficiency in highly extended elastomer formulations and allows free radical cure of the systems with reduced or no surface tack. The amount of chlorinated and/or chlorosulfonated polyethylene as supplemental ingredients in the compositions of the first composition aspect of the invention may be from about 1% to about 50% by weight, preferably 15% to 40% by weight and more preferably from 20% to 35% by weight.
The inclusion of these two polymers as supplemental ingredients in some cases has been found to permit use of lower concentrations of the compositions of the first composition aspect of the invention in formulation of the compositions of the second composition aspect of the invention.
The following examples further illustrate the best mode contemplated by the inventors for the practice of their invention and are to be construed as illustrative and not in limitation thereof.